Excitotoxicity is related to excessive activation of glutamate receptors which results in neuronal cell death. The physiological function of glutamate receptors is the mediation of ligand-gated cation channels with the concomitant influx of calcium, sodium and potassium through this receptor-gated channel. The influx of these cations is essential for maintaining membrane potentials and the plasticity of neurons which in itself plays a pivotal role in cognitive function of the central nervous system. Li, H. B., et al., Behav. Brain Res., 83:225-228 (1997); Roesler, R., et al., Neurology, 50:1195 (1998); Wheal, H. V., et al., Prog. Neurobiol., 55:611-640 (1998); Wangen, K., et al., Brain Res., 99:126-130 (1997). Excitotoxicity plays an important role in neuronal cell death following acute insults such as hypoxia, ischemia, stroke and trauma, and it also plays a significant role in neuronal loss in AIDS dementia, epilepsy, focal ischemia. Coyle, J. T. & Puttfarken, P., Science, 262:689-695 (1993). Neurodegenerative disorders, such as Huntington's disease (HD), Alzheimer's disease (AD), Parkinson's disease (PD) and amyotrophic lateral sclerosis (ALS), are characterized by the progressive loss of a specific population of neurons in the central nervous system. Growing evidence suggests that glutamate-mediated excitotoxicity may be a common pathway which contributes to neuronal cell death in a wide range of neurological disorders. Coyle, J. T. & Puttfarken, P., Science, 262:689-695 (1993).
The molecular mechanisms of excitotoxicity-mediated neuronal cell death remains obscure. Over-production of free radicals that lead to impairment of mitochondrial function is the most widely held hypothesis. Beal, M. F., et al., Ann. Neurol., 38:357-366 (1995); Coyle, J. T. & Puttfarken, P., Science, 262:689-695 (1993). However, it is unclear whether the increase of free radicals is the precursor that initiates neuronal degeneration or, rather, a subsequent consequence of neuronal degeneration. Interestingly, administration of antioxidants has little neuroprotective effect in patients suffering from various neurodegenerative diseases. Shults, C. W., et al., Neurology, 50:793-795 (1998). Thus, some other mechanism(s) must exist for excitotoxicity-induced neuronal cell death.
c-Jun N-terminal kinases (JNKs) are identified as kinases which are activated upon stimulation by various environmental stimuli such as UV light, γ irradiation and mitogenic signals. Hibi, M., et al., Genes Dev., 7:2135-2148 (1993); Kyriakis, J. M., et al., Nature, 369:156-160 (1994). The precise biological function of JNKs remains to be explored. However, some recent reports suggest that JNKs are involved in neuronal apoptosis induced by deprivation of survival factors, i.e., neurotrophic factors which support neuronal survival. Ham, J., et al., Neuron, 14:927-939 (1995).
Mixed-lineage kinases (MLKs), so called because these proteins contain structural domains associated with a variety of cell types, were cloned from a cDNA library derived from mRNA from cancer tissue. MLKs were initially thought to participate in the oncogenesis of some cancers, although high levels of expression of MLKs were found in the normal brain. Dorow, D. S., et al., Eur. J. Biochem., 213:701-710 (1993); Dorow, D. S., et al., Eur. J. Biochem., 234:492-500 (1995).
Searching for biochemical targets which are amenable to screening for neuroprotective therapeutic agents is of central concern in neuroscience today. However, no clinically available pharmaceutical tool to date is employed for blocking excitotoxicity and preventing neuronal cell loss in various neurological disorders due to a lack of suitable biochemical targets. Glutamate receptor antagonists, such as MK-801, although successful in protecting neurons in animal experiments, have all failed in the clinical setting due to their blockage of cognitive function mediated by the receptors, as well as high toxicity to the central nervous system. Thus, an understanding of the molecular mechanism(s) of neuronal cell death induced by excitotoxicity is essential for the identification of new biochemical targets and the establishment of reliable methods for screening new therapeutic drugs from chemical libraries that can be utilized in the treatment of a variety of neurological disorders.